Kelly referred to the "vast tonnages" of marijuana and hard drugs that 'TCOs' — transnational criminal organizations — move across the border from Mexico.

Though Kelly has come out in favor of prosecuting marijuana, he told NBC's "Meet the Press" on Sunday that marijuana "isn't a factor" in the drug war, and is scarcely the biggest issue on the US-Mexico border.

Kelly said that until Congress passes a law to change marijuana's federal status, "we in DHS are sworn to uphold all the laws on the books."

"Additionally, science tells us that it is not only psychologically addictive but can also have profound negative impact on the still developing brains of teens and up through the early 20s," Kelly said.

Kelly pledged to investigate marijuana's "illegal pathways" into the US, and arrest those involved with the drug trade "according to federal law."

Perhaps most tellingly, Kelly said that Immigration and Customs Enforcement, or ICE, would continue to use marijuana possession and distribution convictions as "essential elements" for deportation of undocumented immigrants.

"And let me be clear about marijuana. It is a potentially dangerous gateway drug that frequently leads to the use of harder drugs. Additionally, science tells us that it is not only psychologically addictive but can also have profound negative impact on the still developing brains of teens and up through the early 20s. Beyond that, however, its use and possession is against federal law and until the law is changed by the U.S. Congress we in DHS are sworn to uphold all the laws on the books.

DHS personnel will continue to investigate marijuana's illegal pathways along the network into the U.S., its distribution within the homeland, and will arrest those involved in the drug trade according to federal law. CBP will continue to search for marijuana at sea, air and land ports of entry and when found take similar appropriate action.

When marijuana is found at aviation checkpoints and baggage screening TSA personnel will also take appropriate action. Finally, ICE will continue to use marijuana possession, distribution and convictions as essential elements as they build their deportation / removal apprehension packages for targeted operations against illegal aliens. They have done this in the past, are doing it today, and will do it in the future."